The invention relates to a method of testing the transmission path, within an apparatus of a modular construction, for interruptions, using a test arrangement comprising at least one transmitter, at least one receiver and a control device.
Apparatuses which are used in, for example, contemporary message transmission systems become increasingly more complicated because of the fact that a large number of performance features of such message transmission systems must be made available for the user. A criterion to judge the performance of the message transmission system is its availability to the user.
This availability is defined as the quotient of the average interference-free time MTBF and the sum of the average interference-free time MTBF and the mean value for the time spent in repairing the message transmission system (MTTR). The availability is also the probability, expressed in a percentage figure, of finding a network or a portion of the network, at a predetermined instant, in a state in which it is in working order. The network (transmission system) is the totality of exchanges, terminal stations and transmission arrangements including the transmission paths.
In order to obtain a large number of performance features within the message transmission system, the apparatuses used therein consist of an ever increasing number of modules. Usually this increase results in a lower mean value MTBF. In addition, a large number of modules in the apparatus usually result in longer repair times, increases the mean value MTTR and imposes more stringent requirements on the maintenance staff. Together, the mean values MTBF and MTTR decrease its availability for the user. If specific measures are taken to increase the mean value MTBF, the circuit cost and design effort for the apparatus become larger and, consequently the apparatus becomes more expensive. To reduce the cost of the apparatus and to accomplish an improved availability for the user, efforts are usually made to decrease the repair time, that is to say a decrease of the mean value MTTR. The time spent in repairing the apparatus can be reduced by providing the apparatus with diagnostic units and test arrangements. An important object for such test arrangements used in telephone exchange systems or telephone transmission systems is the testing of lines, through which the message (for example a speech signal) is transmitted, for interruptions.
FIG. 1 shows a prior art test arrangement for testing the transmission path within a modular apparatus, for interruptions in the transmission path. In the apparatus, the test arrangements, which comprise a test signal transmitter, a test signal receiver, a control device and a coupling field, are arranged in the central station. Testing for line interruptions can, for example, be effected by switching the inputs and outputs of the modules to the test arrangement. In small apparatuses, this test arrangement may constitute a comparatively large portion of the total cost and design efforts for the apparatus.
If the change-over of the inputs or outputs of the modules is effected with the aid of relays, the cost for the test arrangement is not only significantly increased by the relays also reduce the means value MTBF of the modules. If the relay is replaced by a different switching device (for example a transistor), it must satisfy very high requirements as regards the symmetry of the transmission path and the overvoltage resistance (of the transistor).